Production rates exceeding three tons of hot metal per cubic meter of working volume per day can now be reached with modern blast furnaces. This was made possible by using improved burden materials, better burden distribution techniques, tighter process controls, very high hot-blast temperatures, oxygen enrichment technology, pulverized-coal injection, and natural gas fuel enrichment. All of which result in much higher average heat loads and fluctuations that would otherwise land inside on the steel containment shells of up-to-date blast furnaces if it were not for stave coolers.
Integrated steelworks use blast furnaces to supply themselves the pig iron they use to make steel. The large gains being made in furnace-productivity have also placed overwhelming demands on cooling system capacities. Liquid cooled stave coolers for blast furnaces were first developed in the late 1960's. Pure copper stave coolers have been used since the late 1970's to thermally insulate the outer steel containment shells of blast furnaces from the intense process heats now being generated in state-of-the-art, high stress furnaces. Copper stave coolers are also capable of delivering furnace campaign lives that now exceed fifteen years.
Where each stave cooler must be positioned within a blast furnace will be a primary determiner of the average thermal load levels it will be subjected to. Cast-iron staves can be successfully used in the less demanding middle and upper stack areas of blast furnaces, but the much higher average heat loads below in the lower stack, Belly, Bosh, Tuyere Level, and Hearth all require the use of higher performing, but more costly copper staves. See FIG. 1.
Cast iron staves are less efficient at cooling than are copper staves because the cast iron metal is relatively much lower in thermal conductivity. Steel pipes in cast iron staves are often coated with material that help with metal bonding. Otherwise, cracks in the cast iron can propagate into the steel pipes. Cast iron staves will also self-generate an insulating layer during operation that acts as a thermal barrier between its internal water-cooling tubes and the cast iron stave body. Both effects reduce the overall heat transfer abilities of cast iron staves.
Such inefficiencies in heat transfer pile up significantly higher cast iron stave hot face temperatures, e.g., over 700° C. Thermal deformations in cast iron staves are hard to avoid when they are overstressed this way. Cast iron stave bodies are further subject to phase-volume transformations when operated at very elevated temperatures. Fatigue cracking, stave body material spalling, and cooling pipes exposed directly to the furnace heat are common failures.
One stave cooler I worked on with Todd Smith is described by him in United States Published Patent Application US-2015-0377554-A1, dated Dec. 31, 2015. Such relates to a “stave comprising an outer housing, an inner pipe circuit comprising individual pipes housed within the outer housing, wherein the individual pipes each has an inlet end and an outlet end and wherein each pipe may or may not be mechanically connected to another pipe, and a manifold, integral with or disposed on or in the housing; wherein the inlet and/or outlet ends of each individual pipe is disposed in or housed by the manifold. The manifold may be made of carbon steel while the housing may be made of copper.” He further adds, “Each of the inlet and outlet ends of each individual pipe may be surrounded in part by cast copper within a housing of the manifold.”
When liquid cooled stave coolers are disposed inside the steel containment shells of smelting furnaces, each conventional coolant connection must have a corresponding penetration or access window in the shell in order to complete the hose connections outside. And, conventionally, each stave cooler must be bolted to or otherwise mechanically attached to the steel containment shell to provide vertical support to itself and the refractory brick lining it cools.
The hot smelting inside the furnaces produces very hot, toxic, and flammable process gases that will find escape paths between the refractory bricks, and between the stave coolers and out through any openings in the containment shell. So these penetration points must have good gas seals. One penetration is easier to seal and keep sealed than several.
But because the stave coolers, containment shells, and refractory brick are all subject to thermal expansion forces, the gas seals can be compromised by constantly being worked back and forth. Stave coolers like those described by Todd Smith, have two or more independent circuits of coolant piping inside, and each produces two coolant connection ends that must passed out back and through the containment shell.
Todd Smith describes a “manifold” that can be made of carbon steel on the back of a housing that may be made of copper. He points out that his stave 100 provides for ease of installation since it reduces the number of access holes or apertures required in the furnace shell 51 necessary for the inlet/outlet piping 108 to and from 100 through furnace shell 51. And he says, at paragraph [0094], that stave 100 is of very strong construction to provide much of the support necessary for installation of the stave 100 on furnace shell 51. The effects of stave expansion/contraction due to temperature changes in the furnace are minimized since individual pipe connections to furnace shell have been eliminated. And, stave 100 reduces weld breaches in pipe connections with furnace shell 51 since such connections have been eliminated. Todd Smith says further that his stave 100 reduces the importance/criticality of any support bolts needed to help support stave 100 on furnace shell 51 since such bolts are no longer relied upon to independently support stave 100 since manifold 106 carries much of the load required to support stave 100 on furnace shell 51.
What has proved to be needed by the industry is a stave cooler that has one-only through-bulkhead always collared in steel, and through which all coolant piping passes from two or more independent coolant circuits in a single rectangular copper body through in a group and connect externally outside the steel containment shell. And which stave coolers depend entirely for their vertical mechanical support by a single hanging of the through-bulkhead in a single corresponding penetration of the containment shell. The effects of stave expansion/contraction due to temperature changes in the furnace must be minimized by tightly grouping the individual pipe connections through the furnace shell to limit the linear ranges possible.